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I will certainly check the heater grounding for V1 2 and 3. This has been modified by someone and I'm pretty sure the earth connection has been removedDavid

The reason I mention trying floating the heaters with a capacitor is possible cathode>>>heater leakage.The capacitor will charge up to a potential and needs to be 600v low leakage.The capacitor charges up and lowers the difference in potential between the two.This is a common fault liked to oscillation via a grounding potential that varies with pot settings.There are other things you can try on the grounding but its a good idea to check this first.It could be the "mods" on the heater grounding are because its on DC but this isolation won't help the potential will vary and modulate the other heater voltages. NB not the heater supply but the potential from heater to ground.

Hi David, I am not familiar with the behavior of the amps you have but something that has happened in other amps...Check the cases of the controls for continuity to ground. I ran into two other amps that would behave like you describe and when the cases of the pots were grounded it went away. Whether they ground to the signal or chassis sometimes matters as well.

OK back in the workshop. The vol pot is decently grounded, so it's not that. The amp stops the oscillation if I disconnect the line input connections to the vol pot. So first move s to reinstate the original 220k resistors that were between the pot input and ground. I guess these were there for a purpose and that purpose was to ensure the input was not open circuit even with no source connected.If the resistors don't sort it then I'll have a look at then put wiring but as this is standard I'm pretty confident it's the resistors.Thanks for all the pointers.David

So 220k resistors installed and no difference. Oscillation stops when I disconnect the input on one channel. There doesn't seem anything strange about the input wiring but I will investigate this further.David

Try inserting 100pF or even 220pF at input to gnd if dosen't work then stop.

It could be that your amp isn't connected to mains ground and your input source is connected to mains ground.If it your current amp is connected to mains ground then check for poor ground wiering, this will cause noise when you input a signal and no more noise when remove.

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